The complete removal and recycling of solvent and unreacted vinyl acetate monomer from poly(vinyl acetate), during the production of poly(vinyl acetate), is a complex and energy-intensive process. However the complete removal of vinyl acetate is particularly important in subsequent processing of the poly(vinyl acetate) to produce poly(vinyl alcohol). For example, residual vinyl acetate in the saponification of poly(vinyl acetate) to produce poly(vinyl alcohol) causes discoloration in the poly(vinyl alcohol).
In order to better understand the problems in removing vinyl acetate from the poly(vinyl acetate), a brief description of a process for producing poly(vinyl acetate) to be used in poly(vinyl alcohol) production is given below.
Poly(vinyl acetate) for poly(vinyl alcohol) production is typically produced by the reaction of vinyl acetate in methanol in stages in a plurality of polymerization zones. However, other solvents for the polymerization reaction, such as tert-butanol, have been reported in the literature. The extent of the reaction in the polymerization zones and the molecular weight of the poly(vinyl acetate) are critically dependent on the ratio of solvent to vinyl acetate. Reactor effluent consists of poly(vinyl acetate) containing solvent and unreacted vinyl acetate, and is referred to as poly(vinyl acetate) paste. Traditionally, the solvent and unreacted monomer have been removed from the poly(vinyl acetate) paste by feeding it into the top of a stripping column. A stream of hot solvent vapor is fed into the bottom of the column to strip the unreacted vinyl acetate from the paste. The paste can be diluted with additional solvent before being fed into the column. A vinyl acetate-containing solvent stream exits from the top of the stripping column while the bottoms, containing poly(vinyl acetate) and solvent and essentially free of vinyl acetate, are fed to saponification units. Since careful control of solvent is required in the polymerization zone to control the molecular weight of the poly(vinyl acetate) for many molecular weight products, solvent must be separated from the vinyl acetate before recycling the vinyl acetate. This involves a complicated distillation process. Therefore, alternative methods of removing and recycling solvent and unreacted monomer from poly(vinyl acetate) have been of continuing interest in the industry.
Other problems in this process scheme relate to the use of the solution of poly(vinyl acetate) in solvent, from the stripping column, in the saponification process. Saponification, as used herein, refers to alkaline-catalyzed transesterification of poly(vinyl acetate) with an alcohol; typically methanol. The presence of residual amounts of unreacted vinyl acetate in the poly(vinyl acetate)/methanol stream which exits the stripping column can cause discoloration of the saponification product. In addition, because of the stripping process, high polymer concentrations are not feasible. However, higher polymer concentrations is sometimes desirable for the saponification process.
The need remains for a method of completely removing residual vinyl acetate after polymerization of the vinyl acetate and of better controlling the ratio of poly(vinyl acetate) to alcohol which is then reacted in a saponification process.
The use of extruders to remove volatiles from polymers is known. For example, there is a description of the use of single-screw and twin-screw extruders in the devolatilization of polymers in Devolatilization of Polymers, edited by Joseph A. Biesenberger (Hanser, 1983), pages 65 to 85. In addition, several patents, such as U.S. Pat. No. 4,198,265 (Johnson, 1980), disclose use of extruders for removing volatiles from polymers. However, incorporation of extruder technology into vinyl acetate polymerization processes in order to recycle solvent and unreacted monomers, without adding more solvent, and to subsequently disperse and dissolve the poly(vinyl acetate) in a hydrolyzing solvent, has not been reported.